The practive of delivering high energy DC shocks to the thorax of patients for the purpose of converting ventricular fibrillation to a hemodynamically stable supraventricular rhythm has been clinically employed for the past 30 years. These shocks, however, can result in MB-creatine kinase release and frank myocardial necrosis. Present energy levels used for defibrillation are empiric and highly variable. This, in part, is due to the convention of quantifying the electrical dose for defibrillation in units of energy; however, current rather than energy defibrillates the heart. In older to individually optimize the magnitude of the electrical shock, it is necessary to characterize the percent of total current delivered to the thorax that actually traverses the heart, the requisite transmyocardial current needed to defibrillate the heart, and the factors that alter these variables. The hypotheses of this study are that: 1) transmyocardial current flow is dependent on the relative resistances of the transthoracic structures, 2) the thorax and intrathoracic structures can be modeled as 3 parallel, variable resistors that can predict the effective transmyocardial current delivered from transthoracic shocks, and 3) defibrillation is dependent on achieving on optimal current density in a ceitical mass of the heart. This proposal will measure transmyocardial current flow and compare this quantity to the total current delivered to the thorax. Transmyocardial current density will be determined in closed-chest dogs by a triaxial electrode system that will measure current density in three dimensions over a 4.0 cm2 region from 12 simultaneous left ventricular sites. The minimal myocardial current density necessary to achieve defibrillation will be determined. The correlation between current density thresholds and prospectively measured subject related parameters that may predict the optimal electrical pulse for each individual will be examined. The information obtained from this study should further elucidate the physiology of defibrillation and therefore provide the basis for more sophisticated methods in prospectively determining the minimal amount of energy (or current) required to defibrillate an individual subject.